From German Patent No. DE 199 12 911 an optical fire alarm is known, which includes a radiation transmitter and a radiation receiver and requires no optical labyrinth, so that it is able to be installed in a room ceiling in a flush manner. The fire alarm also encompasses an arrangement by which, first of all, soiling of the transparent shutter disk of the fire alarm can be detected and, secondly, the proper operation of the radiation transmitter and radiation receiver of the fire alarm provided to detect smoke can be monitored.
From German Patent No. DE 100 46 992 a fire alarm is known which has an arrangement by which smoke and other foreign bodies in the scatter volume can be distinguished.